926 



MOLLUSCA AND BKACHIOPODA 



that their rounded extremities crop out upon the inner surface of the 

 shell in an imbricated (tile-like) manner (a). All true Terebratulidce, 

 both recent and fossil, exhibit another very remarkable peculiarity ; 

 namely, the perforation of the shell by a large number of canals, 



FIG. 700. A, internal surface, a, and oblique section, fc, of shell of Waldheimia 

 australis ; B, external surface of the same. 



which generally pass nearly perpendicularly from one surface to the 

 other (as is shown in vertical sections, fig. 701), and terminate inter- 

 nally by open orifices (fig. 700, A), whilst externally they are covered 



by the periost raciun (B) . Their 

 diameter is greatest towards 

 the external surface, where 

 they sometimes expand sud- 

 denly, so as to become trum- 

 pet-shaped ; and it is usually 

 narrowed rather suddenly 

 when, as sometimes happens, 

 a new internal layer is formed 

 as a lining to the preceding 

 (fig. 701, A, d d). Hence the 

 diameter of these canals, as 

 shown in different transverse 

 sections of one and the same 

 shell, will vary according to 

 ^e part of its thickness wlich 



the section happens to tra- 

 verse. The shells of different 



<V** * perforated Sracto- 

 pods, however, present very 

 striking diversities in the size and closeness of their canals, as shown 

 by sections taken in corresponding parts ; three examples of this 

 kind are given for the sake of comparison in figs. 702-704. These 

 canals are occupied in the living state by tubular prolongations of 

 the mantle, whose interior is filled with a fluid containing minute 

 cells and granules, which, from its corresponding in appearance with 

 the fluid contained in the great sinuses of the mantle, may perhaps 



opening by large trumpet- shaped orifices 

 on the outer surface, and contracting at 



